1. Field of the Invention
The present invention relates generally to a laser transmitter including a semiconductor laser and an external optical resonator formed by a frequency-selective fiber directional coupler.
2. Description of the Related Art
A laser transmitter is disclosed in German Patent Application No. P 36 00 726.9, and in particular in FIG. 2 of the German application and the appertaining descriptive passages. In the disclosed laser transmitter, radiant laser energy is coupled into one waveguide from a semiconductor laser and is supplied to a partially reflecting feedback device. That part of the radiant laser energy transmitted by the feedback device is supplied to a second optical waveguide through a loop-shaped optical waveguide. The output power of the transmitter is taken at the output of the second optical waveguide. The German Patent Application corresponds in part to U.S. pending application Ser. No. 906,503, filed Sept. 12, 1986.
An extremely narrow-band, single-mode operation is achieved with the disclosed laser transmitter, such operation being required for fiber optic communication systems, and in particular for those systems having heterodyne and homodyne reception.
The disclosed laser transmitter has an advantage in that coupling of a resonator and of the fiber system to the semiconductor laser is required at only one side of the semiconductor laser, in contrast to other embodiments f laser transmitters which have a fiber system connected to one side and a resonator coupled to an opposite side of the semiconductor laser. Adjustment problems in the disclosed laser transmitter are considerably alleviated due to the single-sided coupling. It is also easier to provide high mechanical stability in the coupling of the fiber system and of the external resonator.
Dispersive double-core fibers are disclosed in German Published Application No. 34 35 172.
In IEEE Journal of Quantum Electronics, QE-20(1984), pages 486 and following, is disclosed that the line width of a single mode spectrum is inversely proportional to the length of an external resonator.